1. Field of the Invention
This invention pertains generally to semiconductor bonding techniques, and more particularly to a low temperature, in situ, plasma activated wafer bonding apparatus and method.
2. Description of the Background Art
It is well known that direct wafer bonding is an alternative to using organic or inorganic bonding agents for bonding silicon and a number of other semiconductor materials. For example, direct bonding can be facilitated by first activating the surface of the wafer with a base bath (NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 :O, 1:1:5) for silicon and its oxides, or with an acid bath (HC1:H.sub.2 O.sub.2 :H.sub.2 O, 1:1:6) or HF dip for nitrides such as AIN and Si.sub.3 N.sub.4. Plasma exposure is another known technique for activating the surfaces of wafers to be bonded. These surface activation methods render the wafer surfaces hydrophilic and amenable to bonding. After surface activation, the wafers are placed in a spinner where they are rinsed in de-ionized water and spun dry. After this step the wafers are placed surface to surface at which point van der Waals forces pull the two wafers into contact.
The contact bonds which are formed in accordance with conventional wet surface activation are generally weak (less than 0.1 MPa), and not suitable for device processing. This is because the process of oxidation (or corrosion of any kind) upon which high temperature direct bonding of semiconductor materials is based is the result of a two step process: migration of the reacting species to the reaction site, and then the chemical reaction itself. For example, the high temperature oxidation of silicon (T&gt;700.degree. C.) is known to follow linear kinetics initially until the oxide thickness becomes so thick that the atomic transport is the limiting process. In other words, initially Si and O atoms are directly adjacent or are very close. All that is required is the transfer of electrons between the atoms for the reaction to occur. However, as the oxide thickness increases, oxygen atoms must migrate to the unreacted silicon through the oxide layer.
The energy that must be supplied to the "system" to cause the Si and the oxygen to migrate and react is quite large and, as such, this particular reaction is not self sustaining at low temperatures. Therefore, the bonds are typically strengthened by high temperature anneals (T&gt;900.degree. C.) for silicon and its oxides, and moderate temperature anneals (T.about.300.degree. C.) for nitrides. Following the anneals the interfacial bond obtains strengths greater than 1-2 MPa up to a maximum of about 4 Mpa (absolute values depending on test method). This strength is sufficient for further processing such as backthinning, polishing, and micromachining, and the interface is generally free from detectable voids. However, the temperatures required for the annealing step have limited the use of conventional direct bonding techniques to applications wherein the materials to be bonded can withstand the high temperature anneal. Unfortunately, the elevated temperature exposure can have a detrimental effect on implanted or diffused etchstop layers via diffusive broadening.
Therefore, while it is known that wafers can be direct bonded, conventional bonding methods are only effective with high temperature anneals and, further, some materials are unable to withstand such high temperatures. Accordingly, high temperature bonding is limited in its application.
To avoid material damage and problems with thermal mismatching in bonding dissimilar materials, there exists a need for a direct bonding process whereby direct bonding can be effected using a low temperature anneal. In addition, to prevent absorption of water and other contaminates present in air, there exists a need for a process to bond wafers to one another without exposing the wafers to wet environments. The present invention satisfies those needs, as well as others, and overcomes the deficiencies inherent in conventional direct bonding techniques.